There is a trend in a display device using a transistor (e.g., a liquid crystal panel and an organic EL panel) toward a larger screen. As the screen size becomes larger, in the case of a display device using an active element such as a transistor, a voltage applied to an element varies depending on the position of a wiring which is connected to the element due to wiring resistance, which cause a problem of deterioration of display quality such as display unevenness and a defect in grayscale.
Conventionally, an aluminum film has been widely used as a material used for the wiring, the signal line, or the like; moreover, research and development of using a copper (Cu) film as a material is extensively conducted to further reduce resistance. However, a Cu film is disadvantageous in that adhesion thereof to a base film is low and that characteristics of a transistor easily deteriorate due to diffusion of Cu in the Cu film into a semiconductor layer of the transistor. Note that a silicon-based semiconductor material is widely known as a material for a semiconductor thin film applicable to a transistor, and as another material, an oxide semiconductor has attracted attention (see Patent Document 1).
Furthermore, a Cu—Mn alloy is disclosed as an ohmic electrode formed over a semiconductor layer including an oxide semiconductor material containing indium (see Patent Document 2).